The disclosures of Japanese Patent Applications No. 2001-059808 filed on Mar. 5, 2001, No. 2001-074215 filed on Mar. 15, 2001, No. 2001-085662 filed on Mar. 23, 2001, No. 2001-134560 filed on May 1, 2001, No. 2001-188318 filed on Jun. 21, 2001, and No. 2001-327681 filed on Oct. 25, 2001, each including the specification, drawings, and abstract, are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to a combustible-gas sensor which detects a concentration of combustible gas such as hydrocarbons based on a concentration of an intake-oxygen, for example, an intake-oxygen concentration sensor, and to a diagnostic device which determines whether or not there is a malfunction in the intake-oxygen concentration sensor. The invention also relates to an air-fuel ratio control device for internal combustion engines which is equipped with an intake-oxygen concentration sensor and which corrects an amount of fuel to be supplied to an engine on the basis of an output from the intake-oxygen concentration sensor.
2. Description of the Related Art
A known air-fuel ratio control device for internal combustion engines has an air-fuel ratio sensor disposed in an exhaust passage of an engine so as to detect an exhaust-gas air-fuel ratio and is designed to perform feedback control of an amount of fuel to be supplied to the engine such that the detected exhaust-gas air-fuel ratio becomes equal to a predetermined target air-fuel ratio. Such an air-fuel ratio control device measures, for example, parameters regarding the amount of intake gas in an engine (e.g., output from an air flow meter, pressure in an intake passage of the engine, and engine speed). On the basis of a relation that is stored in advance using these parameters, the air-fuel ratio control device calculates a base fuel supply amount (base fuel injection amount) such that the exhaust-gas air-fuel ratio coincides with the target air-fuel ratio. Furthermore, the air-fuel ratio control device is designed to actually supply the engine with fuel of an amount which is calculated by correcting the base fuel supply amount such that the exhaust-gas air-fuel ratio detected by an exhaust-gas air-fuel ratio sensor coincides with the target air-fuel ratio.
If the base fuel injection amount is thus subjected to feedback correction on the basis of the actual exhaust-gas air-fuel ratio detected by the air-fuel ratio sensor, it becomes possible to correct errors in regard to detection by a sensor for detecting parameters regarding the amount of intake gas in the engine (e.g., an air flow meter, an intake pressure sensor, and the like) or errors in fuel injection amount resulting from aging or dispersion among individual products in the actual amount of fuel injected from fuel injection valves. Therefore, air-fuel ratio control can be performed with precision.
However, in the case of an engine having an intake passage in which a purging device for purging evaporative fuel flowing from a fuel tank is disposed, the air-fuel ratio of the engine may temporarily deviate from a target air-fuel ratio during purge of evaporative fuel even if feedback control is performed on the basis of an exhaust-gas air-fuel ratio sensor as described above.
That is, if evaporative fuel (hydrocarbons) is introduced into the intake passage through purge, the engine receives evaporative fuel (fuel vapors) together with intake gas in addition to fuel supplied through injection. Thus, while the fuel injection amount of the engine is controlled on the basis of the exhaust-gas air-fuel ratio, the fuel supply amount of the engine increases temporarily. Therefore, the air-fuel ratio of the engine may deviate from the target air-fuel ratio. If feedback control of the fuel injection amount of the engine is performed on the basis of the exhaust-gas air-fuel ratio in spite of the occurrence of such a deviation, the amount of fuel supplied through purge in the engine is corrected, so that the air-fuel ratio of the engine coincides with the target air-fuel ratio. However, a relatively small gain is set for air-fuel ratio feedback control so as to prevent hunting. Therefore, if purge on a large scale is started abruptly, air-fuel ratio feedback control based on the output from the exhaust-gas air-fuel ratio sensor alone inevitably requires a considerable time until the air-fuel ratio of the engine converges to the target air-fuel ratio.
In order to solve this problem, there has been excogitated an air-fuel ratio sensor in which an intake-oxygen concentration sensor for detecting a concentration of oxygen contained in intake gas is disposed in an intake passage of an engine and which is designed to correct a fuel supply amount of the engine on the basis of an output from the intake-oxygen concentration sensor. In order to solve the aforementioned problem, there has been excogitated a control method which is designed to calculate an amount of evaporative fuel introduced into an intake passage of an engine on the basis of a concentration of oxygen contained in intake gas, namely, on the basis of a detection result obtained from an intake-oxygen concentration sensor that is disposed in the intake passage so as to detect a concentration of oxygen contained in intake gas. If evaporative fuel (hydrocarbons) is introduced into the intake passage, it burns in an oxidative catalyst disposed in an oxygen concentration-detecting portion of the sensor, so that the concentration of oxygen in the vicinity of the detecting portion decreases in accordance with the amount of evaporative fuel consumed through combustion (i.e., in accordance with the concentration of evaporative fuel). Therefore, the air-fuel ratio can be controlled with precision even during purge by calculating a concentration of evaporative fuel (vapors) contained in intake gas on the basis of an output from the intake-oxygen concentration sensor, calculating an amount of vapors supplied to the engine on the basis of an amount of intake air in the engine and the concentration of vapors, and decreasingly correcting a fuel injection amount of the engine by an amount corresponding to the amount of vapors.
For instance, Japanese Patent Laid-Open Publication No. 11-2153 discloses an air-fuel ratio control device of this type.
The device disclosed in this publication is designed to calculate an amount of evaporative fuel contained in intake gas during purge on the basis of an output from an intake-oxygen concentration sensor disposed in an intake passage of an engine, and to decreasingly correct a fuel injection amount of the engine by an amount corresponding to the calculated amount of evaporative fuel.
By thus performing purge control so as to calculate an amount of evaporative fuel contained in intake gas on the basis of an output from the intake-oxygen concentration sensor and decrease a fuel injection amount by an amount corresponding to the amount of evaporative fuel, it becomes possible to perform a direct operation of correction in which the fuel injection amount is reduced by the amount corresponding to the calculated amount of evaporative fuel contained in intake gas. Therefore, if purge control based on the output from the intake-oxygen concentration sensor is performed, much higher precision and much higher responding performance can be accomplished in comparison with the case where purge control is performed through air-fuel ratio control that is based on the output from the exhaust-gas air-fuel ratio sensor. Accordingly, in the case of an engine designed to perform purge control on the basis of an output from an intake-oxygen concentration sensor, it is possible to obtain a stable air-fuel ratio even if purge is performed on a large scale. Therefore, it becomes possible to perform purge on a large scale within a short period. As a result, purging operation can be performed efficiently.
It is true that an air-fuel ratio control device designed to perform purge control on the basis of an output from an intake-oxygen concentration sensor as disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 11-2153 can accomplish high precision as well as high responding performance as described above. However, if an anomaly occurs in the intake-oxygen concentration sensor, the air-fuel ratio of the engine may be destabilized greatly to the extent of causing fluctuations in engine output or a deterioration in the emission properties of exhaust gas.
Namely, if there is an anomaly in the intake-oxygen concentration sensor, the amount of evaporative fuel cannot be calculated precisely during purge control. Moreover, purge control based on the output from the intake-oxygen concentration sensor is designed to detect evaporative fuel contained in intake gas prior to suction of the evaporative fuel into the engine by means of the intake-oxygen concentration sensor and to directly correct a fuel supply amount of the engine. Thus, if an anomaly occurs in the intake-oxygen concentration sensor, it directly affects the fuel injection amount of the engine. Therefore, purge control based on the output from the intake-oxygen concentration sensor causes a problem in that the air-fuel ratio is destabilized more dramatically as a result of the occurrence of an anomaly in sensor output in comparison with the case of normal air-fuel ratio control.
In addition, a driver is usually unaware whether or not purge is being performed. Therefore, even if there is an anomaly in the intake-oxygen concentration sensor during purge control, the driver merely discerns that fluctuations in engine output have become extraordinarily acute. Thus, in the case of repairs, it is necessary to investigate all the causes that could lead to fluctuations in engine output (e.g., fuel injection valves, an exhaust-gas air-fuel ratio sensor, an ignition system, and the like). Ascertainment of the fundamental cause of the fluctuations may require arduous labors.
Furthermore, in the case where fuel injection of the engine is corrected using the intake-oxygen concentration sensor, the output from the intake-oxygen concentration sensor changes greatly owing to environmental changes such as changes in pressure or flow speed.
As is generally known, an intake-oxygen concentration sensor is structured such that a solid electrolyte such as zirconia is sandwiched between two platinum electrodes functioning as a cathode and an anode respectively and that a diffusion rate-determining layer such as a ceramic-coated layer for inhibiting oxygen molecules contained in intake gas from reaching the cathode is formed on the surface of the cathode (i.e., the intake-side electrode). In a state where the intake-oxygen concentration sensor is disposed such that the cathode is in contact with intake gas in the engine and that the anode is in contact with the atmosphere, if a voltage is applied between the cathode and the anode at a temperature equal to or higher than a certain temperature, oxygen-pumping action takes place. That is, oxygen molecules contained in intake gas are ionized on the side of the cathode (i.e., the intake-side electrode), and the ionized oxygen molecules move toward the anode (i.e., the atmosphere-side electrode) in the solid electrolyte and turn into oxygen molecules again on the anode. This oxygen-pumping action ensures that a current proportional to an amount of oxygen molecules moving per unit time flows between the cathode and the anode. However, since the aforementioned diffusion rate-determining layer inhibits oxygen molecules from reaching the cathode, the output current is saturated as soon as it reaches a certain value. The output current cannot be increased thereafter even if the voltage is raised. This saturation current is substantially proportional to the partial pressure (concentration) of oxygen contained in intake gas. Accordingly, the output current substantially proportional to the concentration of oxygen can be obtained by suitably setting the voltage to be applied. This output current is converted into a voltage signal. Thus, the voltage signal proportional to the concentration (partial pressure) of oxygen contained in intake gas can be obtained from the intake-oxygen concentration sensor. In the case where intake gas contains hydrocarbons such as fuel vapors, the hydrocarbons burn on the platinum electrodes, and the concentration of oxygen in the vicinity of the electrodes decreases. Thus, the oxygen concentration sensor outputs a voltage signal proportional to a concentration of oxygen after combustion of combustibles such as hydrocarbons contained in intake gas.
If there is a constant pressure, the concentration of oxygen contained in intake gas is equal to the partial pressure of oxygen contained in intake gas (more precisely, equal to the ratio of partial pressure of oxygen to intake pressure). However, even in the case where the concentration of oxygen is constant, the partial pressure of oxygen contained in intake gas changes in proportion to the intake pressure if the intake pressure changes. Thus, the partial pressure of oxygen can assume different values. On the other hand, the intake-oxygen concentration sensor is designed to detect a partial pressure of oxygen contained in intake gas. Therefore, even in the case where the concentration of oxygen contained in intake gas is held constant, the output from the intake-oxygen concentration sensor changes if the partial pressure of oxygen changes due to a change in intake pressure. That is, the intake-oxygen concentration sensor outputs an oxygen-concentration signal that changes linearly in proportion to the intake pressure even if the concentration of oxygen is constant. In other words, the signal output from the intake-oxygen concentration sensor exhibits so-called pressure dependency. As a result, the intake system undergoes greater fluctuations in pressure and a more substantial decrease in flow speed than the exhaust system that is open to the atmosphere. Therefore, the sensor output tends to be affected thereby. During a transient change in pressure, namely, during an abrupt change in pressure, the sensor output overshoots and does not follow a curve as expected. This causes a problem of deterioration in measurement precision.
The intake pressure in the engine changes depending on the loaded condition of the engine such as engine load or engine speed. Therefore, if the fuel injection amount of the engine is corrected on the basis of the concentration of oxygen contained in intake gas detected by the intake-oxygen concentration sensor, it is necessary to correct the sensor output in accordance with the intake pressure.
In general, correction of a sensor output is performed on the basis of a detected intake pressure of the engine and reference pressure-change characteristics of sensor output which have been calculated in advance according to the kind (type) of a corresponding sensor.
However, even if the concentration of oxygen is constant, the output from the intake-oxygen concentration sensor changes in accordance with the thickness of the zirconia solid electrolyte or the diffusion rate-determining layer mentioned above. The detecting portion of the intake-oxygen concentration sensor is provided with an explosion-proof cover for preventing combustibles contained in intake gas from being kindled through combustion of combustibles such as hydrocarbons on the platinum electrodes. Pores for introducing intake gas into the detecting portion of the sensor are formed in the explosion-proof cover. If these pores change in size within a tolerance, the output from the oxygen concentration sensor also changes correspondingly. Therefore, even among sensors of the same type, the sensor output or the aforementioned pressure-dependent characteristics may be dispersed for reasons of manufacturing tolerance. Thus, if the pressure-dependent characteristics of the sensor output are dispersed among individual products in the case where the output from the intake-oxygen concentration sensor is corrected in accordance with the intake pressure, the concentration of oxygen contained in intake gas cannot be detected precisely even by correcting the sensor output on the basis of the aforementioned reference pressure-change characteristics. This causes a problem of the impossibility of controlling the fuel supply amount of the engine precisely.
For instance, the intake-oxygen concentration sensor deteriorates after longtime use, and develops a tendency to generate an increased output for the same concentration of oxygen. In the case of an engine equipped with a PCV device for ventilating a crank case, intake gas-introducing pores formed in an explosion-proof cover of an intake-oxygen concentration sensor as described above are clogged due to hydrocarbons or oil particles contained in crank-case emission gas that is recirculated into an intake passage from a crank case. This may bring about substantial irregularities in the sensor output.
If such a sensor is subject to a malfunction, the fuel injection amount is corrected on the basis of an output from the sensor that is subject to the malfunction. As a result, the exhaust-gas air-fuel ratio deviates from its target value and causes a problem of deterioration in exhaust emission properties or deterioration in operational performance of an engine. Even if there is a malfunction in a sensor, the sensor output is corrected in the same manner as in the case of a sensor that is in normal operation. Thus, the sensor output deviates more dramatically from its true value. This may cause further deterioration in emission properties or operational performance.
In quest of a solution to the aforementioned problems, the invention provides a device and a method that make it possible to take appropriate countermeasures corresponding to the type of a malfunction in an intake-oxygen concentration sensor by detecting the anomaly in the intake-oxygen concentration sensor at an early stage in the case where purge control is performed by means of the intake-oxygen concentration sensor, to determine exactly whether or not there is a malfunction in the sensor, and to measure a concentration of combustible gas with high precision.
An air-fuel ratio control device for internal combustion engines according to a first aspect of the invention comprises an evaporative fuel concentration sensor, a purging device, a vapor amount calculation portion, an intake-side purge control portion, an anomalous output detection portion, a determination portion, and a sensor anomaly determination portion. The evaporative fuel concentration sensor is disposed in an intake passage of an internal combustion engine so as to detect a concentration of evaporative fuel contained in intake gas. The purging device supplies evaporative fuel in a fuel tank to the intake passage upstream of the evaporative fuel concentration sensor. The vapor amount calculation portion calculates an amount of the evaporative fuel contained in intake gas on the basis of a value detected by the evaporative fuel concentration sensor. The intake-side purge control portion performs intake-side purge control so as to correct a fuel supply amount of the engine on the basis of a value detected by the evaporative fuel concentration sensor while supplying the intake passage with evaporative fuel. The anomalous output detection portion detects an anomaly in engine output on the basis of a parameter regarding engine output. The determination portion determines whether or not the anomaly in engine output detected during the performance of the intake-side purge control has occurred as a result of the intake-side purge control. The sensor anomaly determination portion determines that there is an anomaly in the evaporative fuel concentration sensor if it is determined that the anomaly in engine output has occurred as a result of the intake-side purge control.
If the anomalous output detection portion detects an anomaly in engine output during intake-side purge control on the basis of the parameter regarding engine output, the determination portion determines whether or not the anomaly in engine output results from intake-side purge control. For example, an anomaly in engine output during intake-side purge control may be ascribable to an anomaly in a purge system such as the purging device. Such an anomaly leads to great fluctuations in the amount of evaporative fuel supplied to the intake passage. However, if the evaporative fuel concentration sensor is in normal operation, fluctuations in the amount of evaporative fuel are immediately counterbalanced by correcting the fuel supply amount of the engine. Therefore, the engine output ought to be unaffected. Accordingly, if it is determined that the anomaly in engine output results from intake-side purge control, it is possible to determine that there is an anomaly in the evaporative fuel concentration sensor. In the first aspect of the invention, if it is determined that the anomaly in engine output results from intake-side purge control, the sensor anomaly determination portion determines that an anomaly has occurred in the evaporative fuel concentration sensor. Thus, it becomes possible to take appropriate countermeasures corresponding to a cause of the anomaly, such as cancellation of intake-side purge control based on the evaporative fuel concentration sensor.
An air-fuel ratio control device for internal combustion engines according to a second aspect of the invention comprises an evaporative fuel concentration sensor, a purging device, an intake-side purge control portion, an exhaust-gas air-fuel ratio sensor, an exhaust-side purge control portion, a system anomaly detection portion, and a control change portion. The evaporative fuel concentration sensor is disposed in an intake passage of an internal combustion engine so as to detect a concentration of evaporative fuel contained in intake gas. The purging device supplies evaporative fuel in a fuel tank to the intake passage upstream of the evaporative fuel concentration sensor. The intake-side purge control portion performs intake-side purge control so as to correct a fuel supply amount of the engine on the basis of a value detected by the evaporative fuel concentration sensor while supplying the intake passage with evaporative fuel. The exhaust-gas air-fuel ratio sensor is disposed in an exhaust passage of the internal combustion engine so as to output a signal corresponding to an exhaust-gas air-fuel ratio. The exhaust-side purge control portion performs exhaust-side purge control so as to control an air-fuel ratio of mixture supplied to the internal combustion engine on the basis of a value detected by the exhaust-gas air-fuel ratio sensor while supplying the intake passage with evaporative fuel. The system anomaly detection portion detects an anomaly in a system that is required for the performance of the intake-side purge control. The control change portion cancels the intake-side purge control and starts or continues the exhaust-side purge control if an anomaly in the system is detected.
In the second aspect of the invention, intake-side purge control is canceled if an anomaly occurs in a system required for the performance of intake-side purge control, and purge of evaporative fuel can thereafter be continued through exhaust-side purge control without causing a substantial deviation in air-fuel ratio.
A malfunction determination device for determining whether or not there is a malfunction in an intake-oxygen concentration sensor according to a third aspect of the invention comprises an intake pressure detection portion and a determination portion. The intake pressure detection portion detects an intake pressure of the engine. The determination portion determines whether or not there is a malfunction in the intake-oxygen concentration sensor, depending on whether or not a predetermined relation between amount of change in intake pressure of the engine and amount of change in the output from the intake-oxygen concentration sensor is established when the intake pressure of the engine changes.
That is, the third aspect of the invention makes it possible to determine whether or not there is a malfunction in the sensor, depending on whether or not a predetermined relation is established between amount of change in intake pressure of the engine and amount of change in output from the intake-oxygen concentration sensor.
A combustible-gas sensor according to a fourth aspect of the invention is equipped with a sensor device having a pair of electrodes which are formed on the surface of an oxygen-ion conductor and one of electrodes is disposed in a space where measurement-target gas containing combustible gas and oxygen exists, and detects a concentration of combustible gas on the basis of a change in the concentration of oxygen contained in measurement-target gas resulting from an oxidizing reaction of combustible gas. On the basis of a sensor output in the atmosphere of a reference gas, this combustible-gas sensor corrects a deviation in sensor output resulting from a pressure of measurement-target gas.
The output from the combustible-gas sensor tends to shift to the high-output side as the pressure increases, but the sensor output in a reference gas such as the atmosphere also demonstrates a similar tendency. Therefore, the influence of pressure can be eliminated by performing correction on the basis of such a tendency. Accordingly, it is possible to suppress fluctuations in output resulting from changes in pressure and measure a concentration of combustible gas with precision.
A combustible-gas sensor according to a fifth aspect of the invention corrects a deviation in sensor output resulting from a decrease in flow speed of measurement-target gas on the basis of a map prepared in advance to define a relation between flow speed and sensor output.
The sensor output exhibits flow-speed dependency as long as the flow speed of measurement-target gas is sensibly low, and shifts to the high-output side. Thus, the influence of flow speed can be eliminated by correcting a sensor output on the basis of the map defining the relation between flow speed and sensor output in response to a decrease in flow speed of measurement-target gas.
Furthermore, a combustible-gas sensor according to a sixth aspect of the invention corrects a sensor output on the basis of a pressure-change speed or a rate of change in the concentration of combustible gas during a certain period if the pressure-change speed remains higher than a predetermined speed for the period or more.
The sensor output during a transient change in pressure follows changes in pressure for a certain period since the start of the changes in pressure. After that, however, the sensor output shifts to the low-output side during a decrease in pressure and to the high-output side during an increase in pressure. Therefore, the sensor output is corrected if the pressure-change speed changes abruptly beyond the predetermined speed after the lapse of the aforementioned period. In this case, the relation between pressure-change speed and sensor output or the rate of change in the concentration of combustible gas is calculated in advance. By performing correction on the basis of the relation or the rate of change thus calculated, it becomes possible to suppress fluctuations in output resulting from a transient change in pressure and measure a concentration of combustible gas such as fuel vapors with precision.